The US Centers for Disease Control and Prevention (CDC) released further details today on an outbreak of multidrug-resistant Campylobacter jejuni linked to puppies bought from pet stores.

The outbreak, which began in January 2016, affected a total of 118 people in 18 states through Feb 4, including 29 pet store employees, with 26 hospitalizations and no deaths reported. In an article in today's Morbidity and Mortality Weekly Report, CDC investigators say epidemiologic, laboratory, and traceback evidence indicate that puppies from Petland pet stores and five other pet store chains were the source of the outbreak, and that the findings warrant a closer look at antibiotic use in the commercial dog industry. They also warn of the potential for further cases.

"Although the investigation is completed, the risk for multidrug-resistant Campylobacter transmission to employees and consumers continues," they write.

Strong evidence of dog-to-human transmission

Campylobacter infection is one of the most common causes of diarrheal illness in the United States, affecting an estimated 1.3 million people each year. Most Campylobacter infections are caused by consuming raw or undercooked poultry, but dog-associated infections, though infrequent, have been reported. People can get infected through contact with feces from an ill dog.

The multistate investigation conducted by the CDC and local and state health and agriculture departments found strong evidence linking this outbreak to dogs. In total, 105 of the 106 infected individuals reported dog exposure, including 101 who had contact with a pet store puppy. Ninety-two of those patients reported buying or having contact with a puppy at a Petland pet store, while eight patients reported contact with puppies at five other pet store chains. The investigators say this indicates the dogs probably became infected before reaching the stores.

The link between the human cases and puppy exposure was backed up by whole-genome sequencing of Campylobacter isolates from patients and puppies. The results revealed three different groups of human and puppy isolates that were closely related genetically, suggesting likely transmission from dogs to humans.

To get a better sense of why the Campylobacter infections were drug-resistant, the investigators visited 20 pet stores in four states (Kentucky, Ohio, Pennsylvania, and Wisconsin) and collected antibiotic administration records for 154 puppies. Of the 149 puppies with available information, 142 had received one or more antibiotic courses before arriving or while at the store. Notably, 78 had received antibiotics prophylactically (to prevent illness) and 54 received antibiotics for prevention and treatment. Four antibiotics — metronidazole, sulfadimethoxine, doxycycline, and azithromycin — accounted for 81% of the antibiotics administered.

Information collected for 8 puppies owned by infected patients and 20 puppies with fecal samples that were positive for C jejuni traced the dogs back to 25 breeders and eight distributors—companies that purchase dogs from breeders and then sell them to pet stores. But no single breeder, distributor, or third-party transportation company was identified as the infection source.

Need for antibiotic stewardship in pets

The investigators say the findings suggest that pet stores and clinicians should take heed of the potential for transmission of multidrug-resistant Campylobacter among dogs and from dogs to humans. To reduce this potential, the CDC has shared educational material with retail pet stores focused on hand hygiene, separating human eating areas from animal areas, and use of gloves when cleaning cages. In addition, the article advises clinicians to test for antibiotic susceptibility in human Campylobacter infections.

But given the finding of widespread antibiotic administration among pet store puppies, the investigators say implementation of antibiotic stewardship principles in the commercial dog industry, along with better hygiene and animal husbandry practices to reduce infections, is also needed.

"Antibiotics should only be administered under veterinary supervision with a valid veterinary-client-patient relationship, consistent with existing stewardship principles," they write.

Scott Weese, DVM, a veterinarian and microbiologist who studies bacterial infections in humans and animals at the University of Guelph in Ontario, called the findings "disappointing but unsurprising," explaining that unnecessary antibiotic use in puppies from large breeding operations is often to compensate for poor management and inadequate infection control measures.

"Mass breeding, movement over large distances, and mixing in pet stores creates a lot of opportunities for infection, and too often little is done to try to mitigate that," Weese said in an email. "Yet, a large percentage of those infections are viral or are diseases of management (e.g. diarrhea because of stress or diet), and antibiotics aren't needed.

"If you contrast the antibiotic treatment rates with those from smaller scale breeders, it's clear that this type of management and selling approach isn't good for antibiotic use, dog health, and—as is apparent here—human health."

Although much of the attention in recent years has focused on improving antibiotic stewardship in livestock agriculture, there is increasing acknowledgement of the need to promote judicious use of medically important antibiotics in dogs, cats, and other pets that can be sources of drug-resistant bacteria. In a document released last week, the US Food and Drug Administration said one its goals in the next 5 years will be to develop and implement a strategy for promoting stewardship in companion animals.

"The development of antimicrobial resistant bacteria may impact the ability to effectively treat bacterial infectious disease in companion animals and increase the potential for transfer of antimicrobial resistant bacteria from companion animals to humans through direct or indirect contact," the FDA wrote.